In a single drum stencil printing device, printing ink is supplied to the inner surface of a cylindrical printing drum, and printing paper is pressed onto a stencil master plate sheet wrapped around the outer circumferential surface of the printing drum by using a press roller so that a desired stencil printing by the printing ink may be carried out on the printing paper.
A stencil printing device of this type may be provided with interchangeable printing drums each of which may be selectively mounted on a printer main body so that stencil printing of different colors may be carried out by changing the printing drums. Such a stencil printing device is, for instance, disclosed in Japanese patent publication (kokoku) No. 62-28758.
The printing ink used in such stencil printing devices normally consists of emulsions containing pigments, resin, solvents, surface reactants, and water, and the viscosity of the printing ink is known to depend on temperature. The viscosity of the printing ink affects the amount of ink transferred to the printing paper in the stencil printing process, and this in turn causes a change in the printing density and other printing qualities (such as uniformity in the density of dark areas).
In view of such problems, it has been proposed, for instance in Japanese patent laid open publication (kokai) No. 02-151473, to variably set the pressing force or the pressure of the press roller upon the printing drum in dependence on temperature so that the stencil printing may be carried out with a constant printing density without regard to the change in temperature.
The viscosity of printing ink is affected not only by temperature but also by differences in the compositions and properties of pigments, resins, solvents, surface reactants, and water contents. The color of the printing ink, and the type and particle diameter of pigments also affect the viscosity of the printing ink. Thus, the type (in particular the color) of the printing ink affects the printing density.
Furthermore, the tinting strength (opacity) of each pigment depends on its color or its type, and this also affects the printing density.